This invention relates generally to sand filters, and more particularly to an underdrain assembly for such a filter designed to uniformly and completely distribute backwash water through the filter bed during the backwashing operation.
Sand filters, as known in the art, include filter beds of sand requiring frequent backwashing to free the beds from sludge or other material collected from the water. The backwashing operation is normally carried out by the introduction of clear water below the filter bed under sufficient pressure to thoroughly churn-up the sand and wash out the sludge.
It has been found that underdrains currently used in sand filters for the filtering and backwashing operations fail to uniformly distribute the backwashed water through the sand filter. Dirt or sludge therefore accumulates in the sand bed in those areas where a sufficient supply of backwash water is not made available. Two basic types of underdrains are currently in use, although both have inherent shortcomings.
One type employs a perforated disc and sometimes a cloth-covered plastic disc. The pressure at the top of the disc will vary up to 20 psi during the filter cycle, although the pressure beneath the disc is always less because of the unrestricted flow available to the effluent. While filtering, the pressure forces the sand against the perforations thereby causing a substantial blocking of each opening. Thus, the perforations must be of sufficient number and size to permit an adequate amount of filtered water to pass therethrough regardless of any obstructions formed in the openings. Adequate filtration is therefore made possible, but problems begin with such a filter is backwashed. During backwash, when the water flow is reversed through the underdrain, the perforations thereof are cleared of all obstructions so that water is free to flow through the openings although without any precise control. Naturally, those openings nearest the inlet supplying the incoming water will receive a larger flow than those openings remote from such inlet supply. The result is an uneven flow of water upwardly through the sand bed. Those portions of the filter bed that receive the greater flow will therefore be cleaned more rapidly than those areas where the flow is less. Also, some portions of the filter bed never get backwashed and, as the sand filter ages, its effectiveness is substantially reduced. Another defect of such a filter is that some of the dirt or sludge that had not been backwashed out but was nevertheless dislodged from the sand particles is washed back into the pool when the filter is returned to its filtering position.
Another type of underdrain employs a system including a centrally located hub with straight hollow tubes, having perforations or slits in the periphery, radially projecting from the hub. The problem with such system is that as the tubes extend further from the hub the distance between adjacent tubes increases so that the flow of water through the underdrain during the backwashing operation is greater near the hub and almost non-existant near the outer perimeter of the filter tank. Such a system is therefore incapable of effecting a uniform backwashing operation since a large portion of the sand bed near the outer perimeter of the underdrain receives practically no backwash.